Storage device for storing wafer cassettes for use with a batch furnace

ABSTRACT

A storage device for use with at least one batch furnace for batch treatment of wafers supported in a wafer boat is disclosed. The storage device comprises a cassette storage carousel for storing a plurality of wafer cassettes on rotatable platform stages. A carousel housing bounds a mini-environment chamber in which the platform stages are accommodated. A gas recirculation circuit of the storage device subsequently comprises a gas inlet channel, a gas inlet filter, the mini-environment chamber, a plurality of gas outlet openings in a bottom wall of the carousel housing, a plenum housing bounding a plenum chamber, a plenum chamber outlet, a gas circulation pump connecting the plenum chamber outlet to an inlet end of the gas inlet duct.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patent application Ser. No. 16/765,125 filed May 18, 2020 titled A STORAGE DEVICE FOR STORING WAFER CASSETTES FOR USE WITH A BATCH FURNACE, which is a 371 of International Application No. PCT/NL2018/050791 filed Nov. 26, 2018 titled A STORAGE DEVICE FOR STORING WAFER CASSETTES FOR USE WITH A BATCH FURNACE, which claims the benefit of U.S. Provisional Application Ser. No. 62/590,692 filed Nov. 27, 2017 titled A STORAGE DEVICE FOR STORING WAFER CASSETTES FOR USE WITH A BATCH FURNACE, the disclosures of which are hereby incorporated by reference in their entirety.

FIELD

The invention relates to a storage device for use with a furnace for batch treatment of a batch of wafers which are supported in a wafer boat.

BACKGROUND

U.S. Pat. No. 5,407,449 discloses a batch furnace with a storage device comprising a cassette storage carousel (see FIGS. 1 and 2 ) for storing a plurality of wafer cassettes C which wafer cassettes each accommodate a plurality of wafers W. The storage device may also comprise a wafer transfer robot 14 for transferring individual wafers W between a cassette C that is stored in the cassette storage carousel and a wafer boat B, which is accommodated in a batch furnace station 13 (see FIG. 1 ).

The cassette storage carousel 12 may comprise a carousel housing 16 with at least one cassette transfer opening 18 configured for transferring cassettes C to and from the cassette storage carousel 12. The carousel housing 16 also has at least one wafer transfer opening 20 via which wafers W can be transferred to and from a cassette C which is stored in the carousel housing 16.

In the carousel housing 16 a number of platform stages 22 are accommodated which are connected to a central support which is mounted rotatably around a vertical axis. Each platform stage 22 is configured for accommodating a number of cassettes C. The cassette storage carousel 12 includes a drive assembly that is operatively connected to the central support for rotating the central support with the number of platform stages 22. The carousel housing 16 bounds a mini-environment chamber 24 in which the platform stages 22 are accommodated.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a storage device of the type described above, which is robust, has a low flow resistance and which is easy to manufacture.

To that end, the invention provides a storage device according to claim 1. The storing device comprises a cassette storage carousel for storing a plurality of wafer cassettes constructed and arranged to accommodate a plurality of wafers. The cassette storage carousel comprises:

a carousel housing with at least one opening configured for transferring cassettes or wafers to and from the cassette storage carousel;

at least one platform stage which is connected to a central support which is mounted rotatably around a vertical axis within the carousel housing for accommodating wafer cassettes;

a drive assembly that is operatively connected to the central support for rotating the central support with the at least one platform stage;

a mini-environment chamber which is bounded by the carousel housing and in which the platform stages are accommodated; and

a gas recirculation circuit.

The gas recirculation circuit subsequently comprises:

a gas inlet channel which extends in a vertical direction and is bounded by the central support which is embodied as a central gas inlet duct which is rotatable around the vertical axis, wherein the gas inlet duct has a plurality of gas inlet openings that provide a fluid connection between the gas inlet channel and the mini-environment chamber and which are distributed over the height and the circumference of the gas inlet duct so that, in use, a radially outwardly directed inflow of gas into the mini-environment chamber is provided;

the mini-environment chamber;

a plurality of gas outlet openings which are provided in a bottom wall of the carousel housing a plenum housing bounding a plenum chamber extending under the bottom wall so that the plurality of gas outlet openings provide a fluid connection between the mini-environment chamber and the plenum chamber;

a plenum chamber outlet; and

a gas circulation pump having a gas circulation pump inlet which is connected via a pump inlet duct to the plenum chamber outlet and having a gas circulation pump outlet which is connected via a pump outlet duct to an inlet end of the rotatable gas inlet duct.

Due to the presence of the bottom wall with the plurality of gas outlet openings, in combination with the plenum chamber extending under the mini-environment chamber, a gas recirculation path is created that has a very low flow resistance relative to the known systems while at the same time a rotational symmetric flow within the mini-environment chamber is obtained due to the rotational symmetric radially outwardly directed inflow of gas into via the gas inlet openings in the gas inlet duct which are distributed over the height and the circumference of the gas inlet duct and due to size and the positions of the gas outlet openings in the bottom wall. By virtue of the symmetrical gas flow pattern within the mini-environment chamber, all wafers within the cassettes are surrounded by a substantially laminar flow of particle free, filtered gas so that the chance of contamination of the wafers with particles is minimized. Due to the fact that the plenum chamber is positioned under the bottom wall, the plenum chamber outlet is positioned very close to the inlet end of the gas inlet duct. Thus, the length of the pump inlet duct and the pump outlet duct can be kept very short so that the flow resistances of these ducts, is minimized. The capacity or power of the pump can be kept relatively small when compared with the prior art devices because the flow resistance of the recirculation circuit is relatively small when compared to the flow resistance of the prior art devices.

The present invention also provides a batch furnace assembly comprising:

-   -   a storage device according to the invention;     -   at least one batch furnace station comprising at least one wafer         boat for supporting wafers to be treated in the batch furnace         station; and     -   a wafer transfer robot for transferring individual wafers         between a cassette that is stored in the cassette storage         carousel and a wafer boat of the at least one wafer boat of the         at least one batch furnace station.

The batch furnace assembly according to the invention has the same advantages as those which have been described above in relation to the storage device according to the invention.

Various embodiments are claimed in the dependent claims, which will be further elucidated with reference to an example shown in the figures. The embodiments may be combined or may be applied separate from each other.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic top view of a prior art apparatus disclosed in U.S. Pat. No. 5,407,449;

FIG. 2 shows a perspective view of the cassette storage carousel of the prior art device disclosed in U.S. Pat. No. 5,407,449;

FIG. 3 shows a vertical cross-sectional perspective view of an example of the storage device according to the invention;

FIG. 4 shows another vertical cross-sectional perspective view of the lower part of the example shown in FIG. 3 ;

FIG. 5 shows a horizontal cross-sectional view through the mini-environment chamber showing the bottom wall of the mini-environment chamber;

FIG. 6 shows a horizontal cross-sectional view through the plenum chamber showing the inlet end of the gas inlet duct and the drive assembly; and

FIG. 7 shows a detail VII of FIG. 4 .

DETAILED DESCRIPTION OF THE FIGURES

In this application, similar or corresponding features are denoted by similar or corresponding reference signs. The description of the various embodiments is not limited to the example shown in the figures and the reference number used in the detailed description and the claims are not intended to limit the description of the embodiments, but are included to elucidate the embodiments by referring to the examples shown in the figures.

In most general terms, the invention provides a storage device 10 for storing wafer cassettes C. The storage device 10 may be used with at least one batch furnace station 13 for batch treatment of a batch of wafers which are supported in a wafer boat B. The storage device 10 comprises a cassette storage carousel 12 for storing a plurality of wafer cassettes C constructed and arranged to accommodate a plurality of wafers W.

The cassette storage carousel 12 comprises a carousel housing 16 with at least one opening 18 configured for transferring cassettes or wafers W to and from the cassette storage carousel 12.

The cassette storage carousel 12 comprises at least one platform stage 22 which is connected to a central support which is mounted rotatably around a vertical axis within the carousel housing 16 for accommodating wafer cassettes C. A drive assembly is operatively connected to the central support for rotating the central support with the at least one platform stage 22. The carousel housing 16 may bound a mini-environment chamber 24 in which the platform stages 22 are accommodated.

The storage device according to the invention may comprise a gas recirculation circuit which comprises a gas inlet channel 26 which extends in a vertical direction and is bounded by the central support which is embodied as a central gas inlet duct 28 which is rotatable around the vertical axis L. The gas inlet duct 28 has a plurality of gas inlet openings 30 that provide a fluid connection between the gas inlet channel 26 and the mini-environment chamber 24 and which are distributed over the height and the circumference of the gas inlet duct 28. In use, a radially outwardly directed inflow of gas into the mini-environment chamber 24 may be provided. Then, the recirculation circuit comprises the mini-environment chamber 24 and, subsequently, a plurality of gas outlet openings 34 which are provided in a bottom wall 36 of the carousel housing 16. Subsequently, the gas recirculation circuit comprises a plenum housing 38 bounding a plenum chamber 40 extending under the bottom wall 36 so that the plurality of gas outlet openings 34 provide a fluid connection between the mini-environment chamber 24 and the plenum chamber 40. The plenum chamber has a plenum chamber outlet 42. Finally, the gas recirculation circuit comprises a gas circulation pump 44 having an gas circulation pump inlet 46 which is connected via a pump inlet duct 48 to the plenum chamber outlet 42 and having a gas circulation pump outlet 50 which is connected via a pump outlet duct 52 to an inlet end 28 a of the rotatable gas inlet duct 28.

As already described in the summary section above, the presence of the bottom wall 36 with the plurality of gas outlet openings 34, in combination with the plenum chamber 40 extending under the mini-environment chamber 24, creates a gas recirculation path which has a very low flow resistance relative to the known systems. At the same time a rotational symmetric flow within the mini-environment chamber 24 may be obtained due to the rotational symmetric radially outwardly directed inflow of gas into via the gas inlet openings 30 in the gas inlet duct 28 which are distributed over the height and the circumference of the gas inlet duct 28 and due to size and the positions of the gas outlet openings 34 in the bottom wall 36. Consequently, all wafers W within the cassettes C are surrounded by a substantially laminar flow of particle free, filtered gas so that the chance of contamination of the wafers with particles is minimized.

Due to the fact that the plenum chamber 40 is positioned under the bottom wall 36, the plenum chamber outlet 42 is positioned very close to the inlet end 28 a of the gas inlet duct 28. Thus, the length of the pump inlet duct 48 and the pump outlet duct 52 can be kept very short so that the flow resistances of these ducts 48, 52 is minimized. The capacity or power of the gas circulation pump 44 can be kept relatively small.

In an embodiment, of which an example is shown in the figures, the plenum chamber 40 may extends under the bottom wall 36 over at least 60% of the surface area of the bottom wall 36.

Preferably, the plenum chamber 40 extends over as large a surface area of the bottom wall 36 as possible so that the outflow of gas from the mini-environment chamber 24 to the plenum chamber 40 may be as rotational symmetrical around the vertical axis L as possible. With a coverage percentage of at least 60% a good rotational symmetric flow within the mini-environment chamber 24 can be realized.

In an embodiment, of which an example is shown in the figures (see FIG. 6 ), the plenum housing 38 may have a doughnut shape with an interruption forming a plenum housing recess 38 a. The gas outlet openings 34 which are positioned close to the plenum housing recess 38 a may be bigger than the gas outlet openings 34 which are positioned more remotely from the plenum housing recess 38 a.

The donut shape is substantially rotational symmetric around the central vertical axis L, apart from the plenum housing recess 38 a which is provided to improve the access to the inlet end 28 a of the rotatable gas inlet duct 28. This access is beneficial in view of the service requirements, for example for replacement of gas seals 62, 64, the drive assembly etc. However, the plenum housing recess 38 a might disturb the rotational symmetric gas flow within the mini-environment chamber 24 if no additional measures were taken. Due to the fact that the gas outlet openings 34 which are positioned close to the plenum housing recess 38 a are bigger than the gas outlet openings 34 which are positioned more remotely from the plenum housing recess 38 a, still a substantially rotational symmetric flow pattern can be obtained in the mini-environment chamber 24 even though the plenum housing 38 is not entirely rotational symmetric.

In an embodiment, of which an example is shown in the figures (see FIGS. 4 and 7 ), the storage device 10 may comprise a base frame 54 and a carousel bearing which is embodied a standard ball bearing 56. The standard ball bearing 56 of such an embodiment may have an inner race 58 which is fixedly connected to the inlet end 28 a of the inlet duct 28 and may have an outer race 60 which is fixedly connected to the base frame 54.

U.S. Pat. No. 5,407,449 does not disclose in which way the gas inlet duct 28 thereof is rotatably connected to a base frame. The support of the central duct with the stages may be complicated and require a specially designed and constructed bearing support which is expensive. In contrast, a simple standard ball bearing 56 may be used for supporting the central gas inlet duct 28 and the platform stages 22 which are connected to the gas inlet duct 28 according to the invention. Also the return flow via the plurality of gas outlet openings 34 which are provided in a stationary bottom wall 36 of the carousel housing 16 make it possible to easily provided for rotation of the central support.

In an embodiment, of which an example is shown in the figures, the carousel housing 16 and the plenum housing 38 are connected to the base frame 54.

This embodiment provides a simple, robust and stable construction which is easy to manufacture.

In an embodiment, of which an example is shown in the figures (see FIGS. 4 and 7 ), the storage device 10 may comprise a first gas seal 62 which is substantially annular and provides a substantially gas tight connection between the inlet end 28 a of the rotating gas inlet duct 28 and the bottom wall 36 of the carousel housing 16.

In an embodiment, of which an example is shown in the figures (see FIGS. 4 and 7 ), the storage device 10 may comprise a second gas seal 64 which is substantially annular and provides a substantially gas tight connection between the inlet end 28 a of the rotating gas inlet duct 28 and an outlet end 52 a of the pump outlet duct 52.

In an embodiment, the first and the second gas seals 62, 64 may be the only two gas seals present in the storage device for creating a gas tight connection between the rotating parts of the cassette storage carousel 12 and the non-rotating parts of the cassette storage carousel 12.

In this embodiment, only two gas seals 62, 64 are required to seal off the gas recirculation circuit at positions where a rotating part, i.e. the inlet end 28 a of the gas inlet duct 28, is neighboring a non-rotating part, i.e. the bottom wall 36 for the first gas seal 62 and the outlet end 52 a of the pump outlet duct 52 for the second gas seal 64.

The fact that only two gas seals 62, 64 are required is an improvement. In view of the fact that gas seals are parts which may require replacement when the storage device 10 is serviced, it is beneficial that the number of gas seals present in the storage device 10 between rotating and non-rotating parts can be as low as two. It will be clear that such a low number of gas seals also minimizes the chance of leakage in the gas recirculation circuit.

In an embodiment, of which an example is shown in the figures, the drive assembly may comprise an annular, driven cog wheel 66 which is mounted on the inlet end 28 a of the gas inlet duct 28. Additionally, the drive assembly may comprise a drive motor 68 having a shaft on which a driving cog wheel 70 is mounted. A toothed belt 72 operatively connects the driving cog wheel 70 with the driven cog wheel 66.

A drive with a toothed belt 72 and two cog wheels has the advantage that it does not have play. Consequently, when the drive direction reverses, no shocks or the like will occur. Thus, a smooth, stable and service friendly rotational drive of the central gas inlet duct 28 and the platform stages 22 is obtained. This is beneficial because each disturbance or shock in the system may lead to the release of particles which should be prevented in view of the detrimental effect to the wafers which have to be or have been processed in a batch furnace station 13.

In an embodiment, of which an example is shown in the figures, the storage device 10 may comprise a wafer transfer robot 14 for transferring individual wafers W between a cassette C that is stored in the cassette storage carousel 12 and a wafer boat B. The storage device may further comprise a second mini-environment chamber 74 in which the wafer transfer robot 14 is accommodated. The second mini-environment chamber 74 may be provided with a second mini-environment gas inlet. Additionally, the storage device 10 may comprise a branch duct 80 which branches off from the pump outlet duct 52 and which fluidly connects the pump outlet duct 52 with the second mini-environment gas inlet. The single gas circulation pump 44 supplies gas to both the mini-environment chamber 24 which is bounded by the carousel housing 16 and in which the platform stages 22 are accommodated as well as to the second mini-environment chamber 74 in which the wafer transfer robot 14 is accommodated. An outlet of the second mini-environment chamber 74 may be fluidly connected to the plenum chamber 40.

Due to the very low flow resistance of the gas recirculation circuit in the storage device 10 as disclosed herein, it is even feasible to supply with the same gas circulation pump 44 a second mini-environment chamber 74 in which the wafer transfer robot 14 is accommodated. It is clear, that the solution according to the present embodiment is beneficial in view of operational costs and reduced service requirements. Also, the investment costs for a second gas circulation pump are saved.

In an embodiment, the gas recirculation circuit may comprise a gas inlet filter assembly 32 extending over the plurality of gas inlet openings 30. Such a gas inlet filter assembly 32 serves to minimize the number of particles in the mini-environment chamber 24.

In an embodiment, the plurality of gas outlet openings 34 which are provided in a bottom wall 36 of the carousel housing 16 may have a size and a position configured so that the bottom wall 36 forms a gas flow distribution plate and that, in use, a substantially rotational symmetric flow of gas in the mini-environment chamber 24 prevails.

By choosing the position and the size of the gas outlet openings 34 in the bottom wall 36 in a clever manner, it may even be accomplished that even if the bottom wall 36 is not completely rotational symmetric, for example because over a certain sector the bottom wall 36 is interrupted, still a nice rotational symmetric flow may be created within the mini-environment chamber 24.

In an embodiment, the at least one opening 18 in the carousel housing 16 may comprise at least one cassette transfer opening 18 configured for transferring wafer cassettes C to and from the cassette storage carousel 12. Via this cassette transfer opening 18, wafer cassettes W may be transferred to and from the mini-environment chamber 24 in which the cassette storage carousel 12 is accommodated.

In an embodiment, the at least one opening in the carousel housing 16 may comprise at least one wafer transfer opening 20 via which wafers can be transferred to and from a cassette which is stored in the carousel housing. Via the wafer transfer opening 20, wafers W may be transferred, for example by a wafer transfer robot, to and from a wafer cassette W which is accommodated in the mini-environment chamber on a platform stage 22 of the cassette storage carousel 12.

In an embodiment, the storage device 10 may be configured and arranged to, in use, let prevail a pressure P_(ME) in the mini-environment chamber 24 which is higher than an ambient pressure P_(AM) prevailing in an ambient environment in which the storage device 10 is accommodated.

Thus the ingress of particles into the mini-environment chamber 24 from the ambient environment is prevented.

Finally, the present disclosure provides a batch furnace assembly comprising:

-   -   a storage device according to the invention;     -   at least one batch furnace station 13 comprising at least one         wafer boat B for supporting wafers W to be treated in the batch         furnace station 13; and     -   a wafer transfer robot 14 for transferring individual wafers W         between a cassette C that is stored in the cassette storage         carousel 12 and a wafer boat B of the at least one wafer boat B         of the at least one batch furnace station 13.

The various embodiments which are described above may be used implemented independently from one another and may be combined with one another in various ways. The reference numbers used in the detailed description and the claims do not limit the description of the embodiments nor do they limit the claims. The reference numbers are solely used to clarify. 

The invention claimed is:
 1. A storage device for storing wafer cassettes, the storage device comprising: a cassette storage carousel for storing a plurality of wafer cassettes constructed and arranged to accommodate a plurality of wafers; wherein the cassette storage carousel comprises: a carousel housing with at least one opening configured for transferring the wafer cassettes or wafers to and from the cassette storage carousel; at least one platform stage rotatably around a vertical axis within the carousel housing for accommodating the wafer cassettes; a drive assembly for rotating the at least one platform stage; a mini-environment chamber which is bounded by the carousel housing and in which the at least one platform stage is accommodated; a gas recirculation circuit subsequently comprising: a gas inlet channel which extends in the mini-environment chamber and is bounded by a gas inlet duct, wherein the gas inlet duct has a at least one gas inlet opening that provides a fluid connection between the gas inlet channel and the mini-environment chamber; the mini-environment chamber; a plurality of gas outlet openings which are provided in a bottom wall of the carousel housing; a plenum housing bounding a plenum chamber extending under the bottom wall so that the plurality of gas outlet openings provide a fluid connection between the mini-environment chamber and the plenum chamber; a plenum chamber outlet; a gas circulation pump having a gas circulation pump inlet which is connected via a pump inlet duct to the plenum chamber outlet and having a gas circulation pump outlet which is connected via a pump outlet duct to an inlet end of the gas inlet duct.
 2. The storage device according to claim 1, wherein the plenum chamber extends under the bottom wall over at least 60% of the surface area of the bottom wall.
 3. The storage device according to claim 1, wherein the plenum housing has a doughnut shape with an interruption forming a plenum housing recess, wherein the gas outlet openings which are positioned close to the plenum housing recess are bigger than the gas outlet openings which are positioned more remotely from the plenum housing recess.
 4. The storage device according to claim 1, further comprising: a base frame; and a carousel bearing embodied as a standard ball bearing having an inner race which is fixedly connected to the inlet end of the inlet duct, and having an outer race which is fixedly connected to the base frame.
 5. The storage device according to claim 4, wherein the carousel housing and the plenum housing are connected to the base frame.
 6. The storage device according to claim 1, comprising: a first gas seal which is substantially annular and provides a substantially gas tight connection between the inlet end of the gas inlet duct and the bottom wall of the carousel housing.
 7. The storage device according to claim 6, comprising: a second gas seal which is substantially annular and provides a substantially gas tight connection between the inlet end of the rotating gas inlet duct and an outlet end of the pump outlet duct.
 8. The storage device according to claim 7, wherein the first and the second gas seal are the only two gas seals present in the storage device for creating a gas tight connection between the rotating parts of the cassette storage carousel and the non-rotating parts of the cassette storage carousel.
 9. The storage device according to claim 1, wherein the drive assembly comprises: an annular, driven cog wheel which is mounted on the inlet end of the gas inlet duct; a drive motor having a shaft on which a driving cog wheel is mounted; and a toothed belt operatively connecting the driving cog wheel with the driven cog wheel.
 10. The storage device according to claim 1, further comprising: a wafer transfer robot for transferring individual wafers between a wafer cassette, of the wafer cassettes, that is stored in the cassette storage carousel and a wafer boat; a second mini-environment chamber in which the wafer transfer robot is accommodated, the second mini-environment chamber being provided with a second mini-environment gas inlet; a branch duct which branches off from the pump outlet duct and which fluidly connects the pump outlet duct with the second mini-environment gas inlet, wherein the single gas circulation pump supplies gas to both the mini-environment chamber which is bounded by the carousel housing and in which the platform stages are accommodated as well as to the second mini-environment chamber in which the wafer transfer robot is accommodated.
 11. The storage device according to claim 1, wherein the gas recirculation circuit comprises a gas inlet filter assembly extending over the at least one inlet opening.
 12. The storage device according to claim 1, wherein the plurality of gas outlet openings which are provided in a bottom wall of the carousel housing have a size and a position configured so that the bottom wall forms a gas flow distribution plate and that, in use, a substantially rotational symmetric flow of gas in the mini-environment chamber prevails.
 13. The storage device according to claim 1, wherein the at least one opening in the carousel housing comprises at least one cassette transfer opening configured for transferring the wafer cassettes to and from the cassette storage carousel.
 14. The storage device according to claim 1, wherein the at least one opening in the carousel housing comprises at least one wafer transfer opening via which the wafers can be transferred to and from a wafer cassette, of the wafer cassettes, which is stored in the carousel housing.
 15. The storage device according to claim 1, wherein, in use, a pressure (P.sub.ME) prevails in the mini-environment chamber which is higher than an ambient pressure (P.sub.AM) prevailing in an ambient environment in which the storage device is accommodated.
 16. A batch furnace assembly comprising: a storage device according to claim 1; at least one batch furnace station comprising at least one wafer boat for supporting wafers to be treated in the batch furnace station; a wafer transfer robot for transferring individual wafers between a wafer cassette, of the wafer cassettes, that is stored in the cassette storage carousel and a wafer boat of the at least one wafer boat of the at least one batch furnace station.
 17. The storage device according to claim 1, wherein the at least one gas inlet opening comprises a plurality of gas inlet openings.
 18. The storage device according to claim 17, wherein the gas inlet duct extends vertically in the mini-environment chamber and centrally arranged in the mini-environment, wherein the gas inlet openings of the plurality of gas inlet openings are distributed over the height and the circumference of the gas inlet duct so that, in use, a radially outwardly directed inflow of gas into the mini-environment chamber is provided.
 19. The storage device according to claim 1, wherein the gas inlet duct is embodied as a central support which carries the at least one platform stage, the central support being rotatable around the vertical axis.
 20. The storage device according to claim 19, wherein the central support is operatively connected to the at least one platform stage for rotating the central support with the at least one platform stage. 